1. Field of the Invention
The present invention relates to an improved magnetic head positioning mechanism for disk devices including magnetic disks or optical disks.
The present application claims the priority of Japanese Patent Application No. Heil1-305440 filed on Oct. 27, 1999, which is hereby incorporated by reference.
2. Description of the Related Art
Recording density of magnetic disks is increasing at a rate of 60% or more annually as technology to increase a BPI (Bit Per Inch) and/or TPI (Track Per Inch) improves. In order to implement high BPI devices, in addition to a reduction in an amount of floating of a magnetic head, introduction of the magnetic head with high sensitivity and highly-efficient signal processing, technology of positioning the magnetic head with high accuracy is required.
In a case of the recording density of 1 Gb/in2, for example, density in a direction of a track is 8 kTPI or less and its track pitch is about 3 μm to 4 μm. However, to obtain the recording density of 10 Gb/in2, since its track density has to be 25 kPTI or more and its track pitch has to be lm or less, a magnetic head positioning accuracy of 0.1 μm or less (being equivalent to about 10% of the track pitch) is needed.
FIGS. 14A, 14B, 14C and 15 show a conventional magnetic head positioning mechanism used in magnetic disk devices. As shown in FIGS. 14A and 14B, a magnetic head supporting section (suspension) 5 is composed of a gimbal spring 3 to hold a slider 2 on which a magnetic head is mounted, a load beam 4 to impose a predetermined pressing load on the slider 2 and a base plate 9 and, as shown in FIG. 14C, the magnetic head supporting section 5 is connected, via a boss section 10 formed in the base plate 9, to a holder arm 11 in a caulked state.
Base sections of a plurality of holder arms 11 to hold the magnetic head supporting section 5, as shown in FIG. 14C, constitutes integrally an arm block 12. The arm block 12 holding the magnetic head supporting section 5 is mounted through a rotary bearing section 14 in a magnetic disk device in a manner that it can rotate freely.
As shown in FIG. 15, a movable coil 13 is mounted to an end portion of the arm block 12. A voice coil motor (VCM) is composed of the movable coil 13 and an external fixing magnetic circuit 15 mounted in the magnetic disk device. Such a voice coil motor is adapted to apply a predetermined driving current to the movable coil 13 to generate a driving force which drives the arm block 12 holding the magnetic supporting section 5 to be rotated on a circular arc track in a seek direction (that is, in a direction of a diameter of the magnetic disk) and also drives a magnetic head 1 to perform a positioning operation to find a target track on the magnetic disk (this method is called a “rotary actuator method”).
The positioning operation described herein includes a seek operation (or a tracking operation) to move the magnetic head 1 from an arbitrary track place to a target place and a follow operation to cause the magnetic head 1 to follow the target track.
However, the conventional magnetic head positioning mechanism, since a plurality of magnetic heads 1 is driven simultaneously by one VCM mounted therein, cannot provide sufficient positioning accuracy, especially the track following accuracy in the follow operations required in high TPI positioning devices in which the positioning accuracy of 0.1 μm or less is needed.
To solve this problem, development of a two-stage actuator type magnetic head positioning mechanism is pursued in which each of the magnetic heads 1 is individually driven regardless of driving of the arm block 12 by the VCM.
Japanese Patent Application No. Heil0-355697 with a title “Magnetic head slider positioning mechanism” applied by the present inventor and being pending now, discloses an example of an HGA (Head Gimbal Assembly) two-stage actuator type magnetic head positioning mechanism incorporating piezo-electric elements as shown in FIGS. 16A and 16B.
In the disclosed HGA driving two-stage actuator type magnetic head positioning mechanism, as shown in FIGS. 16A and 16B, a magnetic head supporting section 5 is connected to a tip of an actuator spring 8 and a base portion of the actuator spring 8 is fixed to a holder arm (not shown). A pair of piezo-electric elements 16 is disposed, both being in parallel to each other, on the actuator spring 8, with a center axis of the actuator spring 8 interposed and, while a magnetic head is following a track, a predetermined voltage (for example, ±30V) is alternately applied to each of the piezo-electric elements 16 to generate a driving force which makes flexible both a center spring 18 and side springs 19 mounted on the actuator spring 8, as shown in FIGS. 17A and 17B, and drives the magnetic head supporting section 5 to be rotated minutely in a track direction.
At this point, the piezo-electric elements 16 each being mounted so as to straddle each of driving voids 17, 17, with an “A” portion of each piezo-electric element 16 positioned on a holder arm side (not shown) being fixed as a fixing end, is adapted to expand and shrink a “B” portion of each piezo-electric element 16 positioned on a magnetic head side to make two side springs 19 flexible, which moves minutely the magnetic head supporting section 5 by using a “C” portion in the vicinity of the center spring 18 as a rotation axis.
However, the above HGA driving two-stage actuator type magnetic head positioning mechanism has shortcomings in that, since the driving voids must be mounted on distorting operation portions of the piezo-electric elements 16, not only stiffness in a vertical direction is greatly impaired but also shock-resistance and load/unload durability are decreased. Though required stiffness in the vertical direction can be obtained by changing a geometry of driving spring sections including the center spring 18 or side springs 19 and/or by increasing thickness of a spring plate used for the center spring 18 or the side springs 19 to improve the spring stiffness, it also causes an increase in in-face rotary stiffness, thus leading to a great driving loss at a fine actuator section and to a narrow driving stroke of the magnetic head 1. This makes it difficult to achieve sufficient positioning accuracy and to apply the technology to high TPI positioning devices.
Moreover, another type of the HGA driving two-stage actuator type magnetic head positioning mechanism using piezo-electric elements is available in which driving voids are not formed in the driving portion, as shown in FIG. 18A. This conventional HGA driving two-stage actuator type magnetic head positioning mechanism is so constructed that piezo-electric elements 16 are floated from an actuator spring 8 position due to a thickness of an adhesive layer 24 used to stick both ends of the piezo-electric element 16 to the actuator spring 8, which serves to avoid interference between expanding and shrinking portions of the piezo-electric elements 16 and the actuator spring 8, as shown in FIG. 18B. However, since the thickness of the adhesive layer 24 is as small as about 10 μm, there are risks of contact of the piezo-electric elements 16 with the actuator spring 8 and/or a short-circuit between them.
Furthermore, a same trade-off between the stiffness in a vertical direction and the rotary stiffness occurs structurally in the actuator spring 8 as in a case of the example shown in FIG. 16. That is, if stiffness of a driving spring section is increased to ensure shock-resistance and load/unload durability of the actuator spring 8, a driving loss becomes greater, thus making it impossible to obtain a sufficient moving range, while, if the stiffness of the driving spring section is decreased to minimize the driving loss, it is impossible to ensure shock-resistance and/or load/unload durability.
In the conventional single-actuator-type magnetic head positioning mechanism as shown in FIGS. 14A, 14B, 14C and 15, when the magnetic head supporting section 5 is connected to the holder arm 11, a method is used in which a boss section 10 formed in the load beam 4 is fitted into a mounting hole formed in the holder arm 11 and a swage is inserted with pressure and then caulking is performed. This is because the magnetic head supporting section can be easily positioned and a sufficient connection strength can be obtained. This method is also used widely as a mounting method being excellent in assembly workability, in a case of combining the swage insertion with pressure with application of the adhesive, because a leak of the adhesive to positioning jigs can be effectively prevented by the boss section serving as a wall against the leak.
At this point, since press forming is applied to the boss working, thick materials with plasticity are used. Since the load beam 4 used to produce a pressing load has to be constructed of thin materials with toughness, the boss section 10 formed in a base plate made from materials with plasticity, as shown in FIG. 14A, is junctioned integrally to the load beam 4, a whole of which is connected to the holder arm 11.
In the method of mounting the magnetic head supporting section 5 for the HGA driving two-stage actuator type magnetic head positioning mechanism as shown in FIG. 14A, since the actuator spring 8 having the driving spring section has to be constructed of thin plates with toughness, the press forming is not performed directly on the boss section 10. Therefore, the base plate 9 having the boss section 10 is prepared as a separate member and then the base plate 9 is integrally junctioned to the actuator spring 8, the whole of which is then connected to the holder arm 11. In this method, however, due to an increased number of assembled parts, productivity of the positioning device is decreased and due to increased thickness of the fine actuator section, mounting of the positioning mechanism among narrow plates or parts in a small magnetic disk is made difficult.